Allow explicitly specifying the GCS base URL

[rgehan]

Note

I've opened a PR addressing this issue here

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

I need to build GCS object_store that communicate with a non-standard GCS endpoint (i.e. not https://storage.googleapis.com).

This is already somewhat supported, by reading the gcs_base_url key of a provided service account credentials JSON file (here). I believe this exists for the needs of integration tests mostly.

Unfortunately, in my production use-case, for infra reasons, I cannot provide such a credentials file.

Describe the solution you'd like

I would like to be able to explicitly specify the base URL to be used, as simply as this:

GoogleCloudStorageBuilder::from_env()
  .with_base_url("https://some-other-gcs-endpoint")
  .build();

This is already supported on the AWS S3 object store, see here.

Describe alternatives you've considered

Passing a service account credentials file, but doesn't fit my requirements.

Additional context

None.